Discharge tube oscillator



Feb. 23, 1943.

H. M. BACH 2,311,631

DISCHARGE TUBE 05C ILLATOR Filed Aug. 22, 1940 7 Sheets-Sheet 4 F/GJO.

INVENTOR HE/VR Y M. BA C/-/ ATTORNEY Feb. 23, 1943. H. M. EACH DISCHARGE TUBE OSCILLATOR Filed Aug. 22, 1940 '7 Sheets-Sheet 5 Q W N DI SCHARGE TUBE 0 SCILLATOR Filed Aug. 22, 1940 '7 Sheets-Sheet 6 103 u 107 1 5g 46 4 vvvv 5 7011-? L g Amplifier I| II V II A.V c. AEc.

INVENTOR BYHEA/RY M. BACH zm //M ATTORNEY Feb. 23, 1943. H. M. BACH DISCHARGE TUBE OSCILLATOR Filed Aug. 22, 1940 7 Sheets-Sheet '7 0 1 H 2 2 J C Z K A Q i RE H mm Mm R Mm m 9 m 2 l E y H P M Y a A r 8 w M o 6. 9 R 5 8 6 E 0 a mm fi or A u W 1 mm H" D n n 3 a3 I Patented Feb. 23, 1943 UNITED STATES PATENT OFFICE mesne assignments, to Patents Research Corporation, New York, N. Y., a corporation of New York Application August 22, 1940, Serial No. 353,640

18 Claims.

The present invention relates to an electrical wave generator and among the objects of the invention is to provide an electron discharge tube oscillator for producing sustained oscillations the amplitude and frequency of which may be controlled in an easy and emcient manner.

Another object of the invention isto provide an electron tube oscillator characterized by extreme frequency and amplitude stability.

A further object is to provide an oscillator for converting a direct current to an alternating current of any desired frequency characterized by a complete isolation between the direct current input and the alternating current output circuits.

A further object is the provision of a vacuum tube oscillator whose frequency may be varied directly by a small change in bias potential applied to an electrode of the oscillator tube.

Still a further object is to provide a vacuum tube oscillator which may be easily and efficiently frequency or amplitude modulated by varying a positive bias potential applied to a non-oscillating electrode of the tube.

Another object is to provide a vacuum tube oscillator the frequency of which may be varied by a small change in direct bias p tential applied to a negative electrode of the tube. I

A further object is to provide a low frequency oscillator utilizing a minimum of discharge tubes and parts and providing extreme frequency sta bility and substantially distortionless output.

Still another object is to provide a combination of a radio frequency oscillator and buffer amplifler or frequency multiplier characterized by high operational stability and power output and embodying a single electron discharge tube.

A further object is to provide a radio frequen cy oscillator capable of providing a wide band frequency modulated signal of high stability and requiring low modulating power compared with frequency modulated oscillators known in the prior art.

Still a further object is to provide an electron tube oscillator embodying the piezo-crystal characterized by extreme frequency and amplitude stability. 4

Still a further object is to provide an electron tube oscillator whose o'scillaitng electrodes draw no direct current.

Another object is to provide a first detectoroscillator combination for use in superheterodyne radio receivers wherein the oscillating frequency may be directly controlled by a small direct bias potential applied to the oscillator.

The above and further objects and aspects of the invention will become more apparent from the following detailed description taken with reference to the accompanying drawings forming part of this specification and wherein:

Figure l is a circuit diagram representing a basic oscillator circuit embodying the principles of the invention,

Figure 2 shows a circuit similar to Figure 1 embodying means for controlling the frequency and other characteristics of the oscillator,

Figure 3 is a graph representing frequency as a function of negative grid potential in an oscillator according to Figure 2, 0

Figure 4 is a further graph showing the frequency as a function of the positive voltage applied to the screen grid or accelerating electrode in the oscillator acccording to Figure 1,

Figure 5 is a graph showing the amplitude of the oscillations as a function of the positive screen or accelerating voltage in an oscillating circuit according to Figures 1 and 2,

Figure 6 shows a modification of theoscillator circuit shown in Figures 1 and 2,

Figure '7 is a graph showing the amplitude of the oscillations asa function of the screen grid or accelerating voltage in an oscillator according to Figure 6, 4

Figures 8 and 9 represent oscillating circuits according to the invention embodying a piezoelectric crystal or an equivalent oscillating device to ensure extreme frequency stability,

Figure 10 shows a modification of an oscillator accordingto the invention comprising an oscillator system and buffer amplifier embodied in a single discharge tube,

Figure 11 shows a low or beat frequency oscillator constructed in accordance with the principles of the invention,

Figure 12 is a complete circuit diagram of a superheterodyne receiver embodying a local oscillator and other features of improvement in accordance with the invention,

Figures 13 and 14 illustrate modifications for employing an oscillator according to the invention in the converter stage of a superheterodyne receiver,

Figures 15 and 16 are diagrams of phase or frequency modulated radio transmitters embodying an oscillator according to the invention.

Like reference numerals identify like parts throughout the different views of the drawings.

Referring more particularly to Figure 1, there is shown a basic radio frequency oscillator constructed in accordance with the principles of the invention. Item l represents an electron discharge tube in the example shown a tetrode comprising a cathode II, a control grid ii, an accelerating or screen grid i3 and an anode or plate I. A parallel resonant circuit comprising an inductance l and a variable condenser I6 is con- 22 and 23, respectively. The accelerating, or

screen grid I3 is maintained at a-positive steady potential with respect to the cathode by connecting it to the positive pole of a suitable high tension source indicated by the plus sign in the drawings while maintaining this grid at cathode or ground potential for alternating or oscillating currents by the provision of a suitable by-pass condenser 24.

If in a system aforedescribed an input signal potential e1 having a frequency f is impressed between the grid l2 and cathode H, the electron stream accelerated from the cathode ll through the grid l2 towards the positive screen or ac,- celerating grid l3 will be directly modulated in accordance with the variations of this potential on the input or control grid l2. Most of the electrons will pass through the meshes of the accelerating grid I3 and form a concentrated space charge or virtual cathode in the region between the accelerating grid and the plate I by reason of the decelerating effect of the latter on the electrons. The fluctuations of the density of this virtual cathode or space charge in the rhythm of the variations of the input potential 21 will cause a corresponding current to flow in the return connection between plate and cathode, or in other words, a so-called space charge coupling will exist between the grid circuit and plate circuit of the tube. This induced current, being a displacement current will be phase shifted 90 with respect to the voltage e1 that causes it and will be of the same frequency as the input potential'ei.

Considering now the tuned circuit l'|-l8 connected between the plate I4 and cathode. to be replaced by an ohmic resistance, it is seen that the induced current is in the plate circuit flowing through this resistance will develop a voltage ('2 in phase with the current i2 and, in view of the property of the space charge coupling, this current being a displacement current will be phase shifted by 90 with respect to the space charge fluctuations, i. e.. in turn with respect to the input voltage er. Hence. the voltage eg between the plate i5 and cathode will be 90 phase shifted'with respect to the voltage e1 between the grid l2 and cathode. Now, if the ohmic resistance connected between the plate ll and the cathode is replaced by the parallel tuned circuit ll--l8 as shown, having a resonant frefrequency ,fx, a further phase shift of the potential e2 developed at the plate l4 with respect to ex will occur. If is is equal to ,f or the frequency of the input signal er, the impedance of the resonant circuit |l-i8 will be a pure ohmic resistance and in this case the voltage e2 will be phase shifted by 90 with respect to the voltage 61 as explained in the foregoing. If. on the other (iiihand, A is higher or lower than the frequency ,f of the voltage el an additional phase shift will be produced in either direction due to the properties of the resonant circuit l'|--l8 and accordingly, the phase shift between e: and er will vary above and below the normal phase shift depending on whether Ix is greater or smaller than the frequency f of the voltage e1. If fx is much greater than I or if ,fx is much less than I, the voltage e2 will be in phase or out of phase with respect to the voltage e1 depending on the sense of departure of Ix from f.

It has been shown above that a variation of the space charge in front of a negative electrode will cause a displacement'current to flow through the electrode and its associated circuit resulting in a voltage of the same frequency as the space charge variations being developed across this circuit. Thus, as described in the foregoing, a voltage 21 impressed between the grid 12' and cathode II will result in a voltage e2 between the plate l4 and cathode, the latter being phase shifted with respect to 91 by 90 if the circuit ll-l 8 is tuned to the frequency of the impressed voltage er and varying in either-direction from the normal 90 phase shift in proportion to the departure of the resonant frequency of the circuit il-IB from the frequency of the voltage e1. It has been found that the relationship between the grid l2 and plate ll may be reversed, that is, if an input potential is impressed upon the plate H a corresponding potential will be established on the grid l2 by virtue of the space charge coupling in substantially the same manner as described above but with the essential difference that the 90 phase shift in this casewill be exactly in the opposite direction fromthe 90 phase shift obtained in the case when the input voltage is impressed upon grid l2 and the output potential developed on the plate ll. This effect is obviously due to a second space charge set up in the vicinity of the control grid I! by the electrons after having been repelled and passing the positive grid in inversed sense.

From the foregoing it is seen that if a voltage an is developed across the circuit I 'lll as a result of a voltage at impressed between the grid I2 and cathode II by the action of the space charge or virtual cathode, a similar effect. will be produced in the opposite direction, that is, a current designated as i: will be developed in the circuit l5l6 by the effect of the voltage 22 by virtue of the space charge couplings in both directions between the grid l2 and plate ll. Accordingly, therefore, e2 will develop a current if in the circuit l5l6 and since this current will be phase shifted by 90 with respect to ea in an opposite sense to the 90 phase shift between er and e2, provided the circuits l5-I6 and I'I-ll are tuned to the same frequency, the feedback current 1': will be in phase with the current in the circuit l5l6 thereby tending to augment or maintain the latter. In other words, the feedback voltage er developed by the flow of ii through l5-l6 will be exactly in phase with the exciting voltage er and tend to sustain the latter in such a manner as to enable the circuit to operate as an oscillator for continuous or sustained oscillations having a frequency determined by the resonant frequency of the circuits l5--I6 and il-IO.

The starting or initiation of the oscillations is explained as follows: If either of the circuits Iii-l6 or "-48 is connected and power supplied to the accelerating electrode [3, a damped oscillating voltage e1 will be developed between the grid l2 and cathode H. This voltage will cause a quadrature voltage of the same frequency to be established between the plate l4 and cathode in the manner described hereinabove. This voltage e2 will in turn cause a feedback voltage exactly in phase with the voltage e1 to be developed across the circuit |5-- |6 tending to sustain the initial damped oscillations. Accordingly, therefore, by a proper design of the damping coefficients or Q of the circuits l5l6 and ll-|8 and other characteristics of the system continuous or sustained oscillations will be maintained in the oscillatory circuits either of which may be connected to a suitable utilization circuit.

The maximum amplitude of the oscillations will be a function of the direct bias voltage on the negative oscillating electrodes, for if either electrode is driven positive the virtual cathode in front of that electrode will be weakened or destroyed. Hence, the oscillations will be limited by the bias on the negative electrodes l2 and M, respectively. The bias on these electrodes will also affect the frequency of the oscillations since i the spacing of the virtual cathode from the negative electrodes varies the space charge capacity forming a part of the capacitative reactance of the oscillatory circuits. In a like manner, varying the screen potential will also a'ifect the space.

charge and cause a corresponding frequency vari ation. The amplitude of the oscillations is further dependent on' the Q-value of the resonant or oscillatory circuits. With the Q of the circuits reduced below a critical value, the maintenance of sustained oscillations will cease since the current i2 will not develop a sufficient voltage 422 across the tuned impedance and in turn the feedback voltage at in this case will not have suificient amplitude to sustain the oscillations. It

is found that there is also a critical screen orly similar to Figure 1 but including means for.

adjusting the steady grid bias E1 and the accelcrating or screen potential Esg by the provision of suitable voltage dividers 2B and 25, respectively. D. C. microammeters 26 and 27 are shown inserted in the grid and plate circuits for measuring the D. C. grid and plate current drawn by the oscillating electrodes.

Figure 3 is a graph showing the oscillating frequency f as a function of the negative bias E1 on grid l2 obtained with a circuit shown in Figure 2. As is seen, a straight line is obtained be- .tween the values of about 1 volt and --4 volts for a constant screen voltage E g and a steady negative plate potential E2 (+105 volts and -2.9 volts for the tube used in the experiments conducted by applicant Within the straight line portion of the curve a 30 k. c. frequency shift is obtained and accordingly if the normal bias on the grid 12 is about 2.5 volts a deviation AEi cal by a condenser inserted in the common cathode of 11.5 volts will afford a frequency shift M of :15 k. c. This shift will be linear and equal to about 1 k. e. per 1 6 volt grid bias change. Accordingly, by using an audio voltage of maximum amplitude il volts either phase or frequency modulation may be obtained from this oscillator. Since the plate bias is held constant the radio frequency output voltage present in the plate circuit will be substantially constant. It is found that no D. C. current is drawn by either oscillating electrode as evidenced by the milliammeters 26 and 21 which remained exactly at zero. Y

Figure 4 is a graph showing the frequency f of the oscillations as a function of the screen or accelerating voltage Esg, with the bias voltage on the grid'IZ (E1) and on the plate I4 (E2) being held constant (2.9 volts for the tubes used during the experiments conducted by applicant). This graph also shows a linear relationship between the change in accelerating voltage and the variation in frequency over a substantial operating range.

Figure 5 is a graph showing the peak radio frequency output voltage of the oscillator as measured on a slide back type peak volt meter as a function of the accelerating or positive screen potential Esg. The grid and plate bias potentials E1 and E2 were held constant at'-2.9 volts. The limited amplitude of the output voltage of approximately .3 volt less than the bias voltage on the oscillating electrodes may be explained by the fact that there must exist a minimum negative potential on the oscillating electrodes at all times to ensure a minimum space charge or virexample shown is equal to about 60 volts for the particular tube used in the experiments. Phase or frequency modulation may be effected by varying the D. C. negative bias on one of the oscillating electrodes. I

Figure 6 shows an oscillating circuit smilar to the preceding diagrams wherein the grid I2 and plate M are biased by a common biasing arrangement 28 comprising a resistance by-passed return for the grid and plate circuits.

Figure '7 shows a graph representing the peak 'radio frequency output voltage as a function of the screen potential Egg obtained with a circuit shown in Figure 6. It is found that a change of the screen potential will no longer produce a change in the oscillating fre uency that i the frequency will be substantially constant independently of the screen or accelerating potential.

This may be expla ned in the following manner.

As the screen potential is increased the screen current will likewise increase, and accordingly. a greater voltage drop will be developed in the cathode b as resistor 28. As more voltage is developed across the latter. the grid l3 and plate M will become increasingly negative and accordingly the frequencv variations due to changes of the screen potential will be cancelled by the variations due to change in the negative bias potenthe particular tube used. The curves shown were obtained with the standard type tubes 1852 and GSA'Tavailable on the market and being in wide usage. Thus, the oscillator according to the invention may be utilized to produce an chicient and highly stable amplitude or frequency modulated oscillation for signalling, testing and various other purposes.

As will be evident from the foregoing the present invention discloses a true electron coupled oscillator wherein the oscillations are sustained solely by electron or space charge coupling within a discharge tube. Without this electron or space charge coupling no oscillations could beproduced or sustained. The advantages of this type oscillator are manifold as will further appear from the following.

As is well known, the stability of a radio frequency oscillator is directly 9. function of the Q of the oscillatory circuits in that the higher the Q the greater the stability of the oscillations. Accordingly, therefore, the oscillator proposed by the invention has great advantages over the known oscillators due to the fact that there is no longer any load whatsoever imposed upon the tuned or oscillatory circuits. This is true because the oscillating electrodes draw no direct current whatsoever. In the usual type ofoscillator on the other hand, the plate has a certain definite impedance usually on the order of several thousand ohms and the plate current draws an appreciable direct current. Thisimpedance is shunted directly across the tuned circuit of the oscillator. Furthermore, the grid bias in the known oscillators is obtained by means of a grid leak and condenser by virtue of the fact that the grid is driven positive during part of the oscillating cycle and hence will develop a bias voltage due to direct current flowing through the grid leal: resistance. Accordingly, the grid likewise has a certain definite impedance at the time durlng...whioh it is driven-positive also causing a dampening efiect and consequent reduction of the Q of the oscillatory circuit connected to the grid.

In the oscillator of the type disclosed by the invention, on the other hand, these drawbacks are completely eliminated. Neither of the oscillating electrodes draws any direct current and accordingly since these electrodes are negatively biased and since there is practically no load whatsoever imposed upon the oscillatory circuits the Q of the latter will not be reduced in any manner whatsoever. A further advantage of the oscillator proposed by the invention is the fact that the oscillator tube works more smoothly since the direct current input is completely isolated from the radio frequency circuits, thus further eliminating the needfor elaborate by-pass and filter elements to isolate the direct current tential upon the oscillating electrodes blocking condensers and the like are completely dispensed with. Still a further advantage of the invention is due to the ease of obtalningextremely high output by means of an oscillator-buffer arrangement without in any manner impairing the frequency stability or wave form of the oscillator, that is any type or value of load impedance may be connected in the output circuit without in any way reacting upon and effecting the operation of the oscillator. A further advantage is the fact that the oscillations are developed at the resonant frequency of the tuned tank circuits 55-46 and l'l--l8, rather than at a frequency slightly difierent from the resonant frequency as in known oscillators.

A further advantage is the fact that the invention is readily applicable to tubes commercially available and in wide usage at the present time.

Figure 8 shows a modification of the invention utilizing a quartz crystal 30 shunted by a resistance H to provide a D. C. return for the grid I2 as an oscillatory element connected between the grid l2 and cathode of the oscillator tube. This circuit has the advantage of extreme frequency stability due to the combined properties of the quartz crystal and the circuit according to the invention. The main advantage of an oscillator of this type is the fact that there is substantially no current flowing through the crystal and the voltage developed across the crystal is only of the order of a few volts. Accordingly, heating of the crystal caused by high radio frequency potential and due to high amplitude mechanical vibration is held at a minimum. By suitably limiting the negative .bias on the oscillating electrodes the amplitude of the oscillations may be reduced to a very low value and accordingly the heat generated in the crystal may be held at a minimum. This condition may be easily obtained by reducing the bias resistor in the cathode return lead to the minimum value that will allow stable oscillation. Likewise, the screen potential may be reduced to a very low value and still sustained oscillations may be developed. Resistance 3! may be replaced by a radio frequency choke coil. 7

Figure 9 shows a modification of Figure 8 wherein the crystal 3B and return resistor 3| are connected between the plate and cathode, the operation and function of this circuit being otherwise identical to that of Figure 8.

Referring to Figure 10, there is shown a further modification of the invention in the form of a combined oscillator-amplifier comprising a single discharge tube. This circuit is substan tially similar to the preceding arrangement but difi'ers therefrom in that the plate It or second oscillating electrode is replaced by a grid 32 and that an additional output plate 33 is provided biased at a high positive potential with respect to the cathode and having connected to it a suitable output or load impedance. The screen I3 is constructed to serve both as an accelerating electrode for the electron coupled oscillator as well as a screen to prevent reaction from the plate 33 upon the remaining electrodes of the tube. In an arrangement of this type oscillations will be developed by the oscillator section ll-l2-l3--l2 and associate oscillatory circuits l5-I6 and [1-H in substantially the same manner as described previously, which oscillations will modulate the electron stream flowing to the plate 33 and through an output impedance inserted in the plate circuit. This output impedance may be a resistor, an inductance or a tuned resonant circuit such as a parallel tuned circuit as shown in the drawings comprising a condenser 34 and inductance 35. For small outputs of the order of those required in a superheterodyne receiver the impedance in the plate circuit may be an ohmic resistor in which case the magnitude of the oscillations may be adjusted by varying the value of this resistor or by varying the plate voltage applied to the tube. For higher outputs as required in a transmitter, the load impedance in the plate circuit is advantageously a parallel resonant circuit as shown which latter will offer high impedance to the desired frequency and accordingly, build up a voltage of high magnitude at this frequency. In the latter case the output may be varied by either adjusting the plate voltage or the screen voltage or in any other suitable manner. a harmonic of the oscillations any integral multiple of the oscillator frequency may be segre-,

gated to obtain output oscillations of desired frequency.

The circuit shown in Figure 10 is extremely useful and advantageous inasmuch as it affords a complete isolation between the output utilization circuit and the oscillator proper and as a variation in the output load will not react in any way upon the oscillator, resulting in extreme operational stability and reliability of this system. In order to prevent direct interaction by stray couplings, the oscillatory circuits l--|6, II-IB, and 34-35 are advantageously shielded by suitable grounded screens 36, 31, 38, respectively, in

a manner well understood.

Referring to Figure 11, there is shown a further application of an oscillator proposed by the invention for producing low frequencies utilizing the principle of the well known b'eat oscillator. There are provided for this purpose a pair of high frequency oscillators at least one of which is constructed in accordance with the present invention and whose outputs are combined to pro duce .a lower beat frequency varying according to the relative frequency adjustment of the high frequency oscillators. In the example illustrated both oscillators are of the type as shown in Figure comprising oscillating tubes HI and I0 with corresponding elements and associate circuits denoted by simple and primed numerals, respectively. The oscillator l0 includes an additional control grid 40 arranged 'between the screen 13 and the plate 33 and excited by the output of the oscillator ID by way of a resistance coupling network 4| of known type. The beat oscillations produced in the plate circuit of the tube ID by the modulating or mixing action of the electron screen are developed by a suitable network or filter 42 and impressed upon a utilization circuit by way of coupling condenser 44 and coupling resistor 43. By controlling the irequency'of either of the oscillators preferably by the aid of a variable bias potential in the manner described and shown for instance in Figure 2, it is possible to obtain a highly stable low frequency output current whose frequency may be varied and accurately adjusted to a desired value.

Referring to Figure 12, there is shown a circuit diagram for a superheterodyne radio receiver embodying an oscillator according to the invention to obtain high stability as well as other features of improvement with a minimum of parts and circuit elements. Radio signals received by an antenna. 45 or other input circuit are impressed by way of a tuned radio frequency By tuning the plate impedance to oscillations.

transformer 48 upon the grid 52 and cathode 43 of a first detector-oscillator or electronic mixer tube 41 comprising further a cathode 48, first oscillating grid 49, a screengrid 50, second oscillating grid 5|, suppressor grid 53 internally .connected to the cathode 48 and an anode or plate 54. Item 55 is a biasing network in the cathode circuit to provide suitable negative operating bias for the oscillating grids 43 and 5| as well as the signal input grid 52. The screen grid 50 is constructed in such a manner as to act both as an accelerating electrode for the oscillator section as well as an electrostatic screen between the latter and the signal grid 52 on the one hand-and between the plate 54 and the remaining electrodes of the tube on the other hand. Oscillatory circuits 56 and 51 are connected to the oscillating grids 49 and 5|, respectively,- whereby local oscillations will be generated in the'manner described and the electron stream'p'assing the signal control grid 52 will be initially modulated in accordance with these By the additional control of the electron stream in accordance with the input signal frequency by the action of grid-Hintermediate frequency signals are produced in the plate circuit of the tube which are impressedupon the input of the intermediate frequency amplifler stage 60 by way of the tuned coupling transformer 58. The adjustable elements of the oscillatory circuits 56 and 51 of the input circuit are suitably tracked and ganged as indicated in dotted lines to effect a uni-control in a manner well known to those skilled in the art. The intermediate frequency amplifier 60 in the example shown is of the hexode-diode type comprising a cathode 6|, input control grid 62, screen grid 33, further control grid 64, suppressor grid 55 internally connected to the cathode SI and anode or plate 36. Item 61 is a biasing network in the cathode circuit to provide proper operating bias for the control grids 62 and 64. Grid 34 is connected to ground or cathode through a parallel tuned circuit 68 resonant to the intermediate frequency for the purpose to be described hereafter. The amplified signal current variations at intermediate frequency in the plate circuit are impressed by way of a tuned transformer 69 upon the diode rectifier circuit comprising diode plate II and load resistor 10. The demodulated or audio signal voltage developed by the latter is impressed upon the input of the first triode section of a double triode audio amplifier tube 14 by way of blocking condenser I2 and adjustable grid lead resistance 13. The amplified audio output is impressed upon the input of the second triode section of tube 14 by way of coupling condenser 15 and grid leak resistance IS. The output of the second triode section serves to energize a suitable translating device such as a loud speaker 18 connected to the output circuit of tube 14 by way of audio frequency transformer 11.

There are further shown in Figure 12 means to effect an automatic tuning control by readjusting the frequency of the local oscillator section of the mixer tube 4! whenever the intermediate signal frequency departs from the fixed resonant frequency of the intermediate frequency amplifier. For this purpose a tune correcting or discriminating potential is generated in the output of the intermediate frequency amplifier by the action of the tuned circuit 68 in a manner described in U. S. Patent 2,208,091 to I. Zakarias. The tuned circuit 88 being excited by space charge coupling with the electron stream molulated in accordance with the intermediate frequency signals impressed upon the grid 62 will cause a potential of varying phase to be established upon the grid 84 depending upon the departure of the impressed intermediate frequency from the frequency to which the circuit 68 is resonant. As a result, the plate current will include a direct component varying both in sense and magnitude in proportion to the frequency departure of the intermediate frequency signals from the fixed resonant frequency of the circuit 68. A corresponding discriminating voltage is developed in the plate circuit by means of a net-work comprising series resistance 83 and by-pass condenser 8! as described in the aforementioned patent specification and this voltage is impressed upon the oscillating grid 49 of tube 41. Thus, whenever the intermediate frequency which varies proportionally to the recelved signal frequency deviates from its assigned value, i. -e'., in turn from the resonant frequencyof the tuned circuit 68, a correcting potential will be applied to the grid 49 in such a manner as to reduce this deviation to a minimum or in other words to effect an automatic frequency or tuning control of the receiver.

I There is thus provided in a single tube a high ga'inradio frequency amplifier and a discrimi-,

nator for correcting frequency of radio frequency input voltage.

There are further shown in Figure 12 means for obtaining automatic volume control comprising a low-pass filter 80-42 suitably connected to the diode circuit to derive a potential varying in accordance with the average carrier amplitude and which is impressed upon the signal input grids 52 and 62 of both the mixer tube 41 and the intermediate frequency amplifier 60 in a manner well understood by those skilled in the art. Thus, the receiver described hereinbefore, in addition to the increased operating stability and other advantages inherent in the new type oscillator proposed by the invention, has the further advantage over the standard receiving systems of enabling an automatic frequency control without requiring any additional tubes such as reactor tubes and other circuit elements.

Referringto Figure 13, there is shown a modification of a superheterodyne circuit embodying 4 a localoscillator according to the invention. According to' this embodiment, a separate local oscillator is provided of substantially the same type as shown in Figure 10. The output oscillations are impressed by way of a resistance coupling network H of known design upon the oscillating control grid l0! of an electron mixer tube I05 of known type comprising further a signal input grid I06 excited by the incoming signal oscillations. The intermediate frequency signals developed in the plate circuit of the tube are applied to the intermediate frequency amplifier by way of a tuned transformer I03 in the manner well understood.

Referring to Figure 14, there is shown a circuit embodying a modified composite mixer tube suited for use in superheterodyne receivers. This tube 90 includes a tetrode and a hexode section with a common cathode 9!. The tetrode section comprises control grid 92, screen or accelerating grid 93 and a plate 94, while the hexode section comprises a control grid 95, the screen grid 85 forming a unit with the screen or accelerating grid of the tetrode section, an injector or signal input grid 91 and a plate Hit, The screen grid 96 is designed to act both as an accelerator for the electron coupled oscillator associated with the tetrode section and as a normal screen grid surrounding the input signal grid 97. Oscillatory circuits Nil-402 are connected to the plate 94 and to the control grid 92, respectively, to generate sustained local oscillations in accordance with the invention. The latter are impressed upon the oscillating grid 95 of the hexode section by directly internally connecting the grid 95 with the grid 92, whereby the electron stream accelerated towards the signal input grid 91 'will be initially modulated by the local oscillations'to produce intermediate frequency signals in the' plate circuit in a manner well understood from the foregoing.

Referring to Figure 15, there is shown an embodiment of the invention in a phase modulated transmitter. The oscillator shown is of substantially the same type as described hereinbefore. There is further provided a microphone circuit comprising microphone III, a battery H2 and an audio frequency transformer 1 H for impressing audio potentials upon the oscillating grid 12 to effect a corresponding frequency modulation of the output oscillations in the plate circuit. The latter are transmitted to an antenna H6 or other transmission circuit by way of resonant coupling circuit comprising a variable condenser H4 shunted by an inductance H5. An arrangement as shown in Figure 15 constitutes a most simple and stable low power phase modulated transmitter suitable for communication over relatively short distances. By the insertion of a suitable audio filter across the microphone, the phase modulation may be changed to frequency modulation.

If higher power output is required such as in frequency modulation broadcasting, a system may be employed as shown in Figure I 6. In the latter, the frequency modulated oscillations proof the frequency controlling electrodes of the oscillator, in the example shown the electrode 32, in such a manner as to counteract and reduce the frequency deviations from the assigned carrier frequency to a minimum.

For generating ultra-high frequency oscillations, it is found that the electron transit time causes a change in the phase angle from of the induced current generated by the varying space charge with respect to the voltage e1. Accordingly, it may be found advisable to slightly detune one of the resonant circuits with respect to the other in order to compensate for this phase shift, and to insure that the feedback voltage e: is in exact phase with the voltage :21.

From the foregoing it is seen that this invention involves basically the utilization of two virtual cathodes or concentrated electron space with respect to a space current supply or accelerating electrode being the only electrode carrying a steady direct current. As a result thereof, a mutual space charge coupling exists between said oscillating electrodes whereby sustained electrical oscillations will be maintained in the oscillatory circuits connected to the latter; By controlling either space charge by varying the respective decelerating potentials or by controlling the accelerating potential on the supply electrode the frequency of the oscillations may be varied within a substantial range due ob'- viously to a varying distance or position of the respective space charges in front of the oscillating electrodes resulting in a variation of the space charge capacity forming a part of the efiective tuning reactance of the system.

ing electrode, a source of potential connected between said accelerating electrode and said cathode to maintain an electron space current through said device, a pair of decelerating electrodes located at opposite sides of said accelerating electrode, means for applying biasing potentials between said decelerating electrodes and said cathode to develop a concentrated electron space charge adjacent thereto, and resonant impedance means tuned to substantially the sanre frequency connected between said decelerating electrodes and said cathode.

2. An oscillator comprising an electron discharge tube having a cathodeand an accelerating grid electrode, a source of potential connected between said accelerating electrode and said cathode to maintain an electron space current through said tube, a pair of decelerating electrodes located at opposite sides of said accelerating electrode, means for applying biasing potentials between said decelerating electrodes and said cathode to develop a concentrated electron space charge adjacent thereto, and parallel tuned oscillatory circuits resonant-to substantially the same frequency and connected between each of said decelerating electrodes and said cathode.

3. An oscillator comprising an electron discharge tube having a cathode and an accelerating grid, a source of potential connected between said accelerating grid andcsaid cathode to maintain an electron space current through said tube, a first decelerating grid electrode located between said cathode and said accelerating grid, a further decelerating electrode located at the's'ide of said accelerating grid opposite from said cathode, means for biasing said decelerating electrodes with respect to said cathode to produce a concentrated electron space charge adjacent thereto, and resonant impedance means tuned to substantially th same frequency connected between each of said decelerating electrodes and said cathode 4. An oscillator comprising an electron discharge tube having a cathode and an accelerat ing grid, a source of potential connected between said accelerating grid and said cathode to maintain an electron space current through said tube,

a first decelerating grid electrode located between said cathode and said accelerating grid, 9. further decelerating electrode located at the side 'of said accelerating grid opposite from said cathode, means for biasing said decelerating electrodes with respect to said cathode to produce a concentrated electron space charge adjacent thereto, resonant, impedance means tuned to substantially the same frequency cgnnectcd between each of said decelerating electrodes and said cathode, and means to control the spacing of said electron space charge from said decelerating electrodes for varying the frequency or the oscillations produced.

5. An oscillator comprising an electron discharge tube having a cathode and an accelerating grid, a source of potential connected between said accelerating grid and said cathode to maintain an electron space current through said tube, a first decelerating grid electrode located between said cathode and said accelerating grid, 9. further dc.- celerating electrode located at the side of said accelerating grid opposite from said cathode,

means for biasing said decelerating electrodes with respect to said cathode to produce a concentrated electron space charge adjacent therethe oscillations produced.

6. An oscillator comprising an. electron discharge tube having a cathode and an accelerating grid, a source of potential connected between said accelerating grid and said cathode to maintain an electron space current through said tube, a first decelerating grid electrode located between said cathode and said accelerating grid, a further decelerating electrode located at the side of said accelerating grid opposite from said cathode, means for biasing said decelerating electrodes with respect to said cathode to produce a concentrated electron space charge adjacent thereto. resonant impedance means tuned to substantially the same frequency connected between each of said decelerating electrodes and said cathode, and means for controlling the biasing potential on at least one of said decelerating electrodes to vary the frequency of the oscillations produced.

centrated electron space charge adjacent thereto.

resonant impedance means tuned to substantially the same frequency connected between each of said decelerating electrodes and said cathode, and means for simultaneously controlling in like sense the steady potentials on said accelerating grid and on at least one of said decelerating electrodes.

8. An oscillator comprising an electron discharge tube having a cathode and an accelerating grid, a source of potential connected between said accelerating grid and said cathode to maintain an electron space current through said tube, a first decelerating grid electrode located between said 8 cathode and said'acceierating grid, a further de-' celerating electrode-located at the side of said accelerating grid opposite from said cathode, means for applying biasing potentials between said decelerating electrodes and said cathode to develop a concentrated electron space charge ad- Jacent thereto, and a pair of parallel tuned circuits resonant to substantially the same frequency each connected between one of said decelerating electrodes andsaid cathode. I

9. An oscillator comprising an electron discharge tube having a cathode and an accelerating grid, a source of potential connected between said accelerating grid and said cathode to maintain an electron space current through said tube, a first decelerating grid electrode located between said cathode and said accelerating grid, a further decelerating electrode located at the side of said accelerating grid opposite from said cathode, means for applying biasing potentials between saiddecelerating electrodes and said cathode to develop a concentrated electron space charge adjacent thereto, a piezo-electric crystal element connected between one of said decelerating elec, trodes and said cathode, a high impedance permeable to direct current shunting said crystal, and a parallel tuned circuit resonant to the crystal frequency connected betweenthe other deceleratin'g electrode and said cathode.

10. An oscillator comprising an electron discharge tube having a cathode, an accelerating nected between each of said decelerating. grids and said cathode, and high frequency load impedance means connected to said anode and cathode.

11. An oscillator comprising an electron discharge tube having a .cathode, an accelerating grid and an anode, means for maintaining saidaccelerating grid and said anode at steady positive potentials with respect to said cathode to maintain an electron space current through said tube, a first decelerating grid located between said cathode and said accelerating grid, a further decelerating grid located between said accelerating grid and said anode, means for applying biasing potentials between said decelerating grids and said cathode to produce a concentrated electron space charge adjacent thereto, resonant impedance means tuned to substantially the same frequency connected between each of said decelerating grids and saidcathode, high frequency load impedance means connected between said anode and cathode, and means to substantially prevent capacitative reaction between said anode and said accelerating grid.

12. An oscillator comprising an electron discharge tube having a cathode, an accelerating grid i and an anode, means for maintaining said actain an electron space current through said tube,

a first decelerating grid located between said cathode and said accelerating grid, a further deceleratin grid located between said accelerating grid and saidanode, means for applying biasing potentials between said decelerating grids and said cathode to produce a concentrated electron space charge adjacent thereto, resonant impedancemeans tuned to substantially thesame predetermined frequency connected between each of said decelerating electrodes and said cathode, a parallel tunedload circuit also resonant to said predetermined frequency connected between said anode and cathode, and means to substantially prevent capacitative reaction between said anode and said accelerating grid.

13. A high frequency oscillator comprising an electron discharge tube having a cathode, a space current supply electrode and a pair of oscillating electrodes, oscillatory circuits tuned to subs antially the same frequency connected between each of said oscillating electrodes and said cathode, a source of potential connected between said supply electrode and said cathode, and means for maintaining said oscillating electrodes at negative potentials with respect to said cathode to produce a concentrated electron space charge adjacent thereto and to maintain sustained selfexcited oscillations in said circuits.

l4,.,,A high frequency oscillator comprising an electron discharge tube having a cathode, a space current supply electrode and a pair of oscillating electrodes, oscillatory circuits tuned to substantially the same frequency connected between each of said oscillating electrodes and said cathode, a source of direct current potential connected between said supply electrode and said cathode, means for maintaining said oscillating electrodes at negative'potentials with respect to said cathode to produce a concentrated electron space charge adjacent thereto to maintain sustained selfexcited oscillations in said oscillatory circuits, and means to control the distance of the space charge from at least one of the oscillating electrodes to vary the frequency of the oscillations produced.

15. A frequency modulated oscillator comprising an electron discharge tube having a cathode, a space current supply electrode and a pair of oscillating electrodes, oscillatorycircuits tuned to substantially the same frequency connected between each of said oscillating electrodes and said cathode, a source of direct current potential connected between said supply electrode and said cathode, means for'maintaining said oscillating electrodes at negative potentials with respect to said cathode to produce a concentrated electron space charge adjacent thereto and to maintain sustained self-excited oscillations in said oscillatory circuits, and means comprising a signal source to control the bias potential of at least one of said oscillating electrodes to vary the frequency of the oscillations produced.

16. An electrical oscillator comprising an electron discharge tube including a cathode and a pair of decelerating electrodes, means for developing a concentrated electron space charge in the vicinity of said decelerating electrodes, and resonant impedance means tuned to substantially the same frequency connected between each said decelerating electrodes and said cathode.

17. An electrical oscillatorcomprising an electron discharge tube including a cathode, an accelerating electrode and 'a pair of decelerating electrodes located on either side of said accelerating electrodes, means for developing a concentrated electron space charge in the vicinity of said decelerating electrodes, and resonant impedance means tuned to substantially the same frequency connected between each of said decelerating electrodes and said cathode.

8. An electrical oscillator comprising an electron discharge tube having a cathode, an accelerating electrode, and a pair of decelerating electrodes, means for developing a concentrated electron space charge in the vicinity of said decelerating electrodes, the coupling effected by the space charge from the first to the second decelerating electrode being of opposite polarity to the coupling effected by the space charge from the second to the first decelerating electrode, and resonant impedance means tuned to substantially the same frequency connected between each of said decelerating electrodes and said cathode.

HENRY M. BACH. 

